Optical polarization is an important parameter of an optical signal in various optical devices, systems and applications. The optical polarization of an optical signal can change or be altered by interacting with an optical medium having optical birefringence in which light experiences different refractive indices at different optical polarizations. Fibers, for example, may be optically birefringent and light propagating in such fibers can change its polarization. The birefringence of a fiber may change with time, often randomly with the fluctuations in the operating conditions such as stresses or temperatures in the fiber.
Polarization maintaining (PM) fiber is an example of an optical birefringent material and exhibits high birefringence and supports two discrete polarization modes, HEslow11 and HEfast11, that are along mutually orthogonal slow and fast axes of the PM fiber. The refractive index of the PM fiber for light polarized along the slow axis in the mode HEslow11 is higher than the refractive index of the PM fiber for light polarized along the fast axis in the mode HEfast11. When the light coupled into the PM fiber is linearly polarized along the slow axis of the PM fiber, only HEslow11 mode is excited and the optical polarization of the guided light is maintained along the slow axis; conversely, when the light coupled into the PM fiber is linearly polarized along the fast axis of the PM fiber, only HEfast11 mode is excited and the optical polarization of the guided light is maintained along the fast axis. This characteristics of preserving optical polarization in the PM fiber can be used in various applications, such as fiber optic gyroscopes, integrated optics devices, high-performance interferometer and Polarimetric sensors, quantum key distribution, and fiber lasers. Perturbations to PM fiber, such as stresses exerted on PM fiber, may cause optical coupling or crosstalk between the two orthogonal polarization modes where optical energy of one polarization mode transfers to optical energy of another polarization mode or vice versa.
An optical fiber tends to be subject to bending, forces or stresses in applications. For example, fibers used for an optical network or fiber communication link, such as International Telecommunication Union recommended ITU-T G.652 single-mode optical fiber and cable, would suffer a fiber bend or stress loss which may adversely affect the performance or reliability of the fiber. Such fiber bending or stress could be measured various ways, including using a commercial multiple-wavelength optical time domain reflectometer (OTDR), e.g. at 1310 nm or 1550 nm, to distinguish a bend loss from other types of losses, e.g. broken, connection loss, etc., uses measured different bend losses information at different wavelengths where usually a bend loss is higher at a short wavelength than that of at a long wavelength.